A number of gas sensors are known to the art. For example, U.S. Pat. No. 4,227,984, issued to Dempsey, et al, discloses a potentiostated 3; -electrode solid polymer electrolyte (SPE) gas sensor. The sensor structure disclosed in this patent has catalytic sensing and counter electrodes on opposite sides of a solid polymeric electrolyte membrane. Miniaturization of such a structure is extremely difficult. Long term stability of the electrode-SPE interface is quite poor due to the swelling nature of the SPE. Still further, such a sensor is not readily adapted to the formation of arrays which can measure a number of different gaseous species or which can provide redundancy in the measurement of one or more gaseous species. Further yet, the construction of such sensors is relatively expensive.
Surface conforming substantially planar sensors are also known to the art. For example, M. Koudelka describes a planar "Clark-type" oxygen sensor in "Sensors and Actuators", 9 (1986) 249-258. Also, M. Koudelka and A. Grisel, describe such a planar sensor in "Miniaturized" "Clark-type" "Oxygen Sensor" as reported in Proceedings of Transducers 85 (Philadelphia, Pa., June 1985). The sensor or sensors described by Koudelka and by Koudelka and Grisel are in the nature of planar 2-electrode oxygen sensors fabricated using standard integrated circuit (IC) technology. The sensors consist of a silver cathode and a silver/silver chloride/chloride ion reference anode. The electrodes are in a planar orientation upon a silicon dioxide layer upon a silicon substrate. An electrolytic medium, in the nature of a hydrogel layer, completely covers the electrodes and their surfaces to a substantial depth, generally at least about 40 to 50 microns. A silicon rubber membrane, generally 25 to 50 microns thick, having pores which are porous to oxygen encapsulates the hydrogel layer and the electrodes. While the planar sensor just described has a number of advantages it is not as sensitive as would be desired and does not have as fast a response time as would be desired since the analyte gas must pass through the hydrogel layer to reach the sensing electrode.
The present invention is directed to overcoming one or more of the problems as set forth above.